UEFI Target groundwork: runtime: extract Windows PE globals scan helper

[sparques]

I am back with another attempt trying to get UEFI as a target.

Summary

This PR extracts the PE/COFF globals-scanning logic from src/runtime/os_windows.go into a separate helper file, src/runtime/os_windows_pe.go.

The extracted code includes:

• PE header structs
• section header structs
• module handle storage
• the helper that scans writable PE sections for GC roots

Why

The Windows runtime currently embeds PE parsing details directly inside findGlobals. Pulling that logic into a dedicated helper makes the runtime code easier to follow and isolates the PE-specific implementation detail in one place. For supporting UEFI, the PE stuff is needed isolated from the rest of os_windows.go.

Behavior

This is intended to be a mechanical refactor only. There should be no functional change to Windows behavior.

[deadprogram]

Thanks for working on this @sparques I made one minor comment.

Can you please rebase against the latest dev branch so the tests can run.

[deadprogram]

@sparques looks like you need to run go fmt

make: *** [GNUmakefile:213: fmt-check] Error 1
Unformatted:
  src/runtime/os_windows_pe.go

[sparques]

@deadprogram Okay, I think it's good now.

[deadprogram]

Thanks for working on this @sparques now squash/merging.